Hydroxy-substituted azobenzenes,

Slow thermally isomerizing azo derivatives are exceptional building blocks for information storage devices. By contrast, for other applications, like optical oscillators and fast-responding artificial muscles, which will be described in the next sections of this chapter, it is essential that the return to the thermodynamically stable tmns form in the dark occurs as fast as possible. Specifically, hydroxy-substituted azobenzenes, commonly known as azophenols, are particularly attracting chromophores for this purpose since they are endowed with fast thermal isomerization rates under ambient conditions. 

The photoisomerization of various substituted azobenzenes, azonaphthalenes and higher aryl-substituted derivatives as well as the role of hydrazone formation and the effect of intramolecular hydrogen bond formation in hydroxy derivatives have also been investigated. 

Many substituted azobenzenes belong to the azobenzene type, as well. Substitution may shift the n —> it band from 440 nm (azobenzene), to 465 nm (hexamethylazobenzene), to 480 nm (2,2 -dimethyl-4,6,4 ,6 -tetra-tert.butylazobenzene), and even to 520 nm (hexaphenylazobenzene). Hydrocarbon, halogen, nitro, carboxy, acetyl, hydroxy, m-amino" and even 2,2 4,4 ,6,6 hexaphenyl substitution— influences the (7i,7t )-(n,7i ) state energy gap, but not so much as to shift the molecule into the aminoazobenzene group. In the context of this book, it is important that the long-chain and polymeric molecules containing azobenzene units coupled by means of hydroxy and carboxy substituents are of the azobenzene type. 

In the presence of metal salts, aryldiazo compounds undergo various coupling reactions in aqueous media. Such reactions have been used extensively in the dye and pigment industry and in anal d-ical chemistry. For example, the AICI3-catalyzed diazo couphng of three different 3-substituted-l//-pyrazol-2-in-5-ones in water with seven different aryldiazonium salts yielded 21 different (5-hydroxy-3-substituted-lFf-pyrazol-4-yl)-azobenzene derivatives as colored products with high yields (Eq. 11.67). ... 

The MCC oxidation of substituted anilines in ACOH-H2O to corresponding azobenzenes is fractional order in substrates and H+ ions. The rates are retarded by electron-withdrawing substituents and enhanced by electron-releasing substituents. The PPCC oxidation of a-hydroxy acids in the same solvent is fractional order in H" " ion and the rates increase with decrease in dielectric constant. The order of the reactivity is mandelic > lactic > glycolic acid. ... 

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